Introduction

- East Asian Monsoon and paleoclimatic data analysis:a vegetation point of view

The past 21 000 years are a very interesting time period pe-
riod as it contains two extreme states of the climate. The
Last Glacial maximum (LGM, 21 000 years BP) is a cold and
generally dry period driven by enlarged ice sheets and low at-
mospheric CO
2
. The mid-Holocene period (6000 years BP),
generally warmer and wetter than the present one, is consid-
ered as orbital forced period with perihelion in northern sum-
mer/autumn and greater-than-present axial tilt (Berger, 1978)
but free of major ice-sheet and relatively high CO
2
(taking as
reference the pre-industrial present time). These two peri-
ods have been chosen as key time periods by the Palaeocli-
mate Modelling Intercompraison Project, PMIP (Joussaume
and Taylor, 1995). The mid-Holocene, with its high summer
insolation, is a period of high land-sea contrast and conse-
quently enhanced monsoon (Braconnot et al., 2002). It is of
particular interest for climate modellers to test their simula-
tions through palaeodata from the monsoonal regions.
The East Asian monsoon (EAM) is one of the most ac-
tive components of the global climate system, influencing a
large area of China and its surrounding countries. In China
and surrounding countries, a megathermal period was recon-
structed from 9500 to 4000 yr ago (Shi et al., 1993). How-
ever, many recent studies have shown that Holocene climatic
changes were asynchronous across China (An, 2000; An
et al., 2006; He et al., 2004). The Holocene optimum was de-
fined as EAM precipitation maximum, occurring ca. 11 000­
9000 yr ago in northeastern China, 11 000­8000 yr ago in
north-central and northern east-central China, 8000­6000 yr
ago in central China, and ca. 3500 yr ago in southern China
(An, 2000). The reason for debate on Holocene climatic
variations is that complexity of the EAM, and different re-
sponses of environmental proxies to climatic changes (Wei
and Gasse, 1999; Wang et al., 2003).
Therefore, more precisely dated palaeo-records and improved quantitative re-
construction are required to provide quantitative insights into
the processes of climatic changes, and their links to the
EAM.

The goal of this paper is threefold. First we explore the
temporal variability of a record located in a sensitive re-
gion at the northern edge of the EAM using a multiproxy
approach. Second we explore the spatial variability of the
Chinese climate at 6 ka BP, when EAM is assumed to be the
strongest. Third we illustrate a new methodology of climate
reconstruction based on vegetation model inversion.
The temporal study is based on a core sampled in Lake
Bayanchagan (Inner Mongolia) (Jiang et al., 2006) (Fig. 1).
This region is particularly sensitive to climate variations as
it is located at the edge of the present EAM. Their results
suggest that this region was dominated by steppe vegeta-
tion throughout the Holocene, except for the period 9200
to 6700 yr BP, when forest patches were relatively common.
This period can then be correlated to enhanced EAM. But
these findings need to be confirmed by a multiproxy analysis.
We will synthesise in the first part of this paper an statistical
approach based, in addition to pollen, on isotopic data and
concentration of a green algae species (Jiang et al., 2008)

This study will focus on the timing of this enhanced EAM
period.
This approach based on detailed time series in a sensitive
region will be completed by a spatial analysis based also on
pollen data but done with the newest tools involving a pro-
cess model able to relate vegetation and climatic variations
(Luo et al., 2008
2
). The strong feature of this approach is
to be able to take into account the large differences existing
between present and mid-Holocene conditions as (i) climate
seasonality, possibly resulting in lack of modern analogues,
or (ii) atmospheric CO
2
close to pre-industrial concentration
but significantly lower than the present one. This spatial anal-
ysis will be first replaced in the context of previously pub-
lished data syntheses at the sub-continental scale.